i) Field of the Invention
This invention relates to a method for manufacturing a clad material by means of indirect extrusion.
ii) Description of the Related Art
As shown in FIG. 7, for example, when conventional indirect extrusion processing is performed, an extrusion tool comprised of a die 3 for defining an outer shape of a product and a mandrel 4 for defining an inner shape of the product is installed inside a container 1, and a billet 2 is set inside the container 1 and thrust against a loose dummy 6. This conventional method of indirect extrusion is disclosed in the Unexamined Japanese Patent Publication No. 9-201618.
In this case, the loose dummy 6 is stationary, and the die 3 and the mandrel 4 are forced together to move relatively toward the billet 2 (more particularly, toward the loose dummy 6) set inside the container 1. Then, the billet 2 is extruded through a die opening 11 into a product shape to form an extruded material 10.
In such a processing method by means of indirect extrusion, the billet 2 is not moved with regard to the container 1, and no friction is generated between an inner wall of the container 1 and the billet 2. Accordingly, less formation of dead metal is achieved, and thus the method has been in the limelight in the field of manufacturing of extruded products with high precision.
Although the above example is for indirect extrusion of a tube, indirect extrusion of a stick (solid material) can be also performed in the same manner only by removing the mandrel 4.
When a clad billet composed of a core material and a coating material undergoes indirect extrusion according to the aforementioned prior art technique, however, dead metal composed of the core material is formed, though it is little, and the core material in the dead metal is extruded as a surface layer of the product at an early stage of extrusion. Accordingly, a defective clad called three-layer clad is formed which is composed of the core material, coating material and core material in layers.
In other words, referring first to FIG. 8A, a die 3 is forced to move relatively toward a billet 50 which is set inside the container 1 and composed of a core material 52 and a coating material 54 coating the outer surface of the core material 52. Then, referring to FIG. 8B, a dead metal 52a area composed of the core material 52 which fails to flow into the die opening 11 is formed in the vicinity of the die 3, although the area is narrow. As the die 3 is further forced to move, referring to FIG. 8C, the core material 52 in the vicinity of the dead metal 52a area is extruded through the die opening 11 as a product surface layer 62. As a result, an extruded material 60 makes a defective clad (three-layer clad) comprising a first layer composed of the core material 52 (product surface layer 62), a second layer 64 composed of the coating material 54 and a third layer 66 composed of the core material 52 (refer to FIGS. 8C and 8D which is a cross sectional view taken along a line 8D—8D of FIG. 8C).